In an ICP optical emission spectrometer, a sample is introduced into a plasma flame so as to emit light through excitation. The thus-emitted light is dispersed through a grating so as to be detected by a photodetector, and as a result, an emission spectrum is acquired. In addition, an element contained in the sample is qualitatively analyzed by the type of wavelength in the spectrum line (bright line spectrum) that appears in the emission spectrum, and furthermore, the element is quantitatively analyzed by the intensity of this bright line spectrum (see Patent Document 1).
FIG. 5 is a schematic diagram showing the structure of an example of a conventional ICP optical emission spectrometer. An ICP optical emission spectrometer 200 is provided with a plasma torch 18 for optical emission spectrometry from which a plasma flame 22 is generated, a sample gas-supplying unit 44, a plasma gas-supplying unit 41, a cooling gas-supplying unit 42, a light measuring unit 43 for detecting the emitted light, a high-frequency power supply 130 for plasma that supplies a high-frequency current I, and a computer (control unit) 150 for controlling the entirety of the ICP optical emission spectrometer 200.
The plasma torch 18 for optical emission spectrometry is provided with a sample gas tube 11 in cylindrical form, a plasma gas tube 12 in cylindrical form that covers the outer periphery of the sample gas tube 11 with a space in between, a coolant gas tube 13 in cylindrical form that covers the outer periphery of the plasma gas tube 12 with a space in between, and a high-frequency inductive coil 21 with two to three loops around the end portion of the outer periphery of the coolant gas tube 13.
The plasma gas-supplying unit 41 allows argon gas to flow in the upward direction at a relatively low speed between the outer periphery of the sample gas tube 11 and the inner periphery of the plasma gas tube 12. As a result, argon gas is jetted from the upper end portion of the flow path created between the outer periphery of the sample gas tube 11 and the inner periphery of the plasma gas tube 12. When the jetted argon gas is ionized by the electrons that have been accelerated by the high-frequency electromagnetic field created by the high-frequency inductive coil 21, argon cations and electrons are generated. The generated electrons further collide with argon so as to proliferate the ionization, and thus, a stable plasma flame 22 is generated in the upper end portion.
The cooling gas-supplying unit 42 allows the argon gas to flow in the upward direction at a relatively high speed between the outer periphery of the plasma gas tube 12 and the inner periphery of the coolant gas tube 13. As a result, argon gas is jetted from the upper end portion of the flow path created between the outer periphery of the plasma gas tube 12 and the inner periphery of the coolant gas tube 13, and the thus-jetted argon gas flows in the upward direction along the outside of the plasma flame 22 that has been generated in the upper end portion.
When a sample is analyzed, the sample and the argon gas are made to flow in the upward direction through the space surrounded by the inner periphery of the sample gas tube 11. The sample is jetted from the end portion of the sample gas tube 11 together with the argon gas so as to be introduced into the plasma flame 22. As a result, a compound included in the sample makes contact with the plasma flame 22 and is converted to an atom or is ionized so as to emit light through excitation.
The light measuring unit 43 has a housing 43a, a condenser lens 43b for introducing the light emitted from the plasma torch 18 for optical emission spectrometry into the housing 43a, a grating 43c for dispersing the emitted light, and a photodetector 43d for detecting the emission spectrum.
The computer 150 is formed of a CPU 151 and input apparatuses 52, such as a keyboard and a mouse, and carries out a qualitative analysis on an element contained in the sample on the basis of the type of wavelength of the bright light spectrum in the emission spectrum detected by the photodetector 43d, and furthermore carries out a quantitative analysis on the element on the basis of the intensity of the bright light spectrum.
The above-described ICP optical emission spectrometer 200 is provided with a high-frequency power supply 130 for plasma that supplies a high-frequency current I to the high-frequency inductive coil 21. The plasma high-frequency power supply 130 is provided with a housing 131 having openings 131a and 131b, a high-frequency circuit substrate 132 placed inside the housing 131, and a cooling fan 133 placed in proximity to the opening 131a of the housing 131.
The housing 131 is in rectangular parallelepiped form having a space inside (50 cm×20 cm×35 cm, for example) where the opening 131a is created at the bottom while the opening 131b is created at the top.
The high-frequency circuit substrate 132 consists of two substrates (30 cm×20 cm×1.6 mm, for example) made of FR4 (Flame Retardant Type 4, thermal conductivity: 0.33 W/mk) in plate form, and transistors, large-scale capacitors and amplifying circuits, which are elements for supplying a high-frequency current I to the high-frequency inductive coil 21, are mounted on the upper surface the substrates. In addition, conventional grease (thermal conductivity: 0.9 W/mk) is applied to the upper surface of the substrates. The high-frequency circuit substrate 132 is placed in the center portion inside the housing 131.
The cooling fan 133 is placed in proximity to the opening 131a in such a manner that the absorbing side is on the bottom and the exhaust side is on the top. In addition, when the fan rotates, the air is allowed to pass from the opening 131a of the housing 131 to the opening 131b of the housing 131 through the inside of the housing 131.
In the thus-formed high-frequency power supply 130 for plasma, elements on the high-frequency circuit substrate 132 emit heat when a high-frequency current I is supplied, and therefore, the cooling fan 131 is rotated so as to allow air to flow, and thus, the heat generated from the elements on the high-frequency circuit substrate 132 is radiated.
In some other ICP optical emission spectrometers, a matching box is provided between the high-frequency power supply 130 for plasma and the high-frequency inductive coil 21 so as to form a structure for reducing the waves reflected from the high-frequency inductive coil 21, and the impedance is matched by changing the capacitance by means of the matching box.